Pseudomonas aeruginosa is an important opportunistic pathogen of humans that is notorious for being the principal cause of morbidity and mortality in Cystic Fibrosis (CF) patients. In the chronically infected CF lung the organism persists as a biofilm[unreadable]a surface attached community of bacteria encased in a polymeric matrix. This biofilm mode of growth augments the resistance of P. aeruginosa to antibiotics and facilitates evasion of the host immune response. Prominent amongst those genes that play an important role in biofilm formation in P. aeruginosa are the cupA genes, which encode components of a putative fimbrial structure that facilitates surface- attachment. We have found that MvaT, a protein recently identified in P. aeruginosa as a global regulator of virulence gene expression, controls the phase-variable (i.e. ON/OFF) expression of the cupA fimbrial gene cluster. The proposed studies will determine how MvaT exerts this control. Our recent findings also implicate Rho, a transcription termination factor, in this same process. We therefore propose to explore the possibility that MvaT and Rho function synergistically to achieve efficient repression of phase-variable cupA gene expression. Moreover, we will attempt to elucidate the mechanism by which phase-variable cupA gene expression (the first of its kind documented in P. aeruginosa) actually occurs. We anticipate that these studies will illuminate the mechanism by which MvaT controls the expression of the cupA genes and, by extension, other MvaT-target genes that contribute to virulence. We anticipate further that these studies will also shed light on the molecular basis behind phase-variable gene expression in P. aeruginosa.